Improvements

Several improvements

- Travis is used to run the tests suite against Python 2.7 and 3.3
- Use a `Makefile` to centralize useful dev commands
- Improve the `README` to display the travis status badge

I'd like to add a lot of other improvements, like rewrite several parts to use Python best practices. And those changes would bring incompatibility with previous versions of the sdk. Do you have some current plans for that ? or can I go my way ?

Cheers